Optical shaft encoders are well known for their use in determining the angular position of a rotating shaft. Such optical shaft encoders are used in various types of machinery and machine tools where information concerning the precise angular relationship or speed between a shaft and another component is needed.
Optical shaft encoders generally consist of an optical shutter, such as a disk or drum, which is rigidly attached to a shaft whose position or speed of rotation is to be determined. The optical shutter is used to modulate the transmission of light between a light emitter (e.g. a light emitting diode) and a light detector (e.g. a photodiode or phototransistor). This modulation may be transmissive, in which case the optical shutter has a pattern of slots formed on its surface and the light emitter and light detector are arranged on opposite sides of the optical shutter.
In most such arrangements, the output of the light detector produces a series of pulses indicative of the passage of the slots borne by the optical shutter past the light emitter and light detector due to rotation of the optical shutter. The frequency of these pulses is indicative of the angular speed of rotation of the optical shutter and its associated shaft.
In some environments it is desirable to know not only the angular speed of rotation of the optical shutter and shaft but also the direction of rotation. For example, in an electricity meter, such as a watthour meter or a demand meter, the direction of rotation of the eddy disk of the meter depends upon the direction of power flow through the meter. In such meters, it is common to use a so-called pulse initiator to detect the motion of the eddy disk due to power flow. In one common type of pulse initiator, a simple pattern of light and dark areas are formed on a single concentric track around the rim of the eddy disk. A light source and light detector are used to detect the presence or absence of a particular marking. A pulse is output every time a predetermined marking is detected. Depending upon the number of such markings around the rim of the eddy disk, a predetermined number of pulses are generated by the pulse initiator to indicate one disk revolution. This information is then used to calculate the amount of electrical demand, i.e. power consumed, for display by a register.
As shown in U.S. Pat. No. 4,321,531 it is known to provide such a pulse initiator with means for determining the direction of rotation of the eddy disk. This is done through the use of a single light emitter and two light detectors arranged to image consecutive portions of the single patterned track formed about the rim of the eddy disk. The direction of rotation of the disk is determined by detecting the order in which the first and second light detector outputs change.
However, this arrangement suffers from the drawback that in order to eliminate ambiguity in the pattern of the outputs of the light detectors, and hence the determination of the direction of rotation of the eddy disk, a particular output of the pulse initiator is validated only after both detectors have seen the same state, that is both detectors must have consecutively seen either a light reflective or a light absorptive area before observing one of the code markings. In practice, this means that the code markings must be spaced sufficiently far apart from each other (e.g. 180.degree.) so that the light detectors will simultaneously see long stretches of the reflective area of the eddy disk. This severely limits the amount of angular resolution with which the pulse initiator can determine the angular speed of rotation of the eddy disk. This drawback is especially evident where the disk moves very slowly or is in the process of changing direction.
This prior art pulse initiator arrangement also requires that the eddy disk be specially adapted for use as an optical encoder. However, it is often desired to retrofit such an optical encoding mechanism to an electricity meter which, heretofore, has required the costly and time consuming replacement of the eddy disk with an optically encoded version and addition of the associated light emitter and light detectors.